Return collection instead of enumerable and fix/re-enable task scheduler check (💥 BREAKING CHANGES)

[cretz]

What was changed

• Change converter/codec/etc non-streaming IEnumerable return values to IReadOnlyCollection
  • Lazy enumerables for this case were confusing users and are against best practices
  • 💥 BREAKING CHANGE on some return types
• Fix issue that inadvertently disabled the task-on-workflow-scheduler checker (i.e. re-enable it)
  • 💥 BREAKING CHANGE on behavior because for the last couple of alphas this checker was inadvertently disabled
• Provided workaround for Task.WhenAny sometimes using thread pool
  • Updated README and made Workflow.WhenAnyAsync that is workflow safe

[bergundy]

Does this method require the Async suffix? Wouldn't it be better to match the Task API naming?

[cretz]

I chose Workflow.DelayAsync even though Task.Delay, so I did the same here. I think following VSTHRD200 everywhere is better than trying to match Task name exactly for the couple of alternatives we're providing.

[bergundy]

I'd make exceptions here just because we're providing alternatives for known APIs.

[cretz]

I don't think we should (and didn't when we introduced DelayAsync). In fact, I suspect .NET regrets not suffixing these with Async back when they wrote it.